I believe the number was somewhere like 50% of action for all online sportsbooks comes from the US. You will also see sports forums close as well due to the revenue stream they are no longer allowed to access as under the new law those sites become illegal. This is one big mess. The big books will find a loophole in this. Most feel that depositing my certified check will be ok and withdrawing by checks will be fine but still it's a bit of a hassle. According to some reports alot of the sportsbooks are trying to get there governments to go to the WTO on this law but the US has ignored the WTO in these situations in the past. It's the fucking religious right who has alot of the voting power causing this whole mess. The Republicans are desperate after there numbers have dropped off and they feel by appeasing the religous right they can in turn get there senators and congressman voted in this November.
I'm a member of a US based fixed forum and I know the admins on the site are extremely concerned to the fact that you alluded to - most of the revenue from the site actually comes in the form of sponsorship from other sportsbooks, even moreso than client money
Interesting times ahead....